Electromagnetic device having a metal oxide varistor core

ABSTRACT

Electromagnetic devices such as inductors and autotransformers having improved suppression to sparking or high voltage peaks at the terminals and between coil windings are provided using certain sintered metal oxide insulators having varistor characteristics. The metal oxide can be employed in conjunction with magnetic core material, as a composite core in such electromagnetic devices.

United States Patent H91 1111 3,710,187 Harnden, Jr. [451 Jan. 9, 1973 s41 ELECTROMAGNETIC DEVICE 3,307,074 2/1967 Lockie ..317/15 HAVING A METAL OXIDE VARISTOR CORE Inventor: John D. Harnden, Jr., Schenectady,

Assignee: General Electric Company Filed: Sept. 30, 1971 App]. No.: 185,269

US. Cl ..317/15 Int. Cl. ..l-l02h 7/04 Field of Search ..3l7/l5; 336/233, 234, 212, 336/219; 174/140 R, 140 C References Cited UNITED STATES PATENTS 8/1957 Hespenheide ..336/2l9 Primary Examiner-James D. Trammell Assistant Examiner--l-Iarvey F endelman Attorney-Frank L. Neuhauser et al.

Electromagnetic devices such as inductors and autotransformers having improved suppression to sparking or high voltage peaks at the terminals and between coil windings are provided using certain sintered metal oxide insulators having varistor characteristics. The metal oxide can be employed in conjunction with magnetic core material, as a composite core in such electromagnetic devices.

10 Claims, 6 Drawing Figures ABSTRACT ELECTROMAGNETIC DEVICE HAVING A METAL OXIDE VARISTOR CORE The present invention relates to the use of certain sintered metal oxide material for improved spark and arcing suppression in various electromagnetic devices.

Prior to the present invention, electromagnetic devices such as inductors and autotransformers were often subject to severe arcing problems resulting from sudden surges of voltages which reduced the service life and performance of such devices. In addition, such electrical devices often suffered from over heating due to the poor thermal conductivity of the core structure.

The present invention is based on the discovery that sintered metal oxide material or varistor can be used for minimizing voltage flash-over and sparking, and as a useful thermal conductor to reduce excessive heating in inductors and autotransformers. The valuable results achieved by using such metal oxide varistors is based on the fact that the metal oxide material exhibits nonlinear resistance characteristics which can be described by the following relationship:

where V is the voltage between two points across the metal oxide material, I is the current flowing between the two points, C is a constant and a is an exponent greater than 1. C and a are functions of the shape of the body and the composition of the body, where C is principally a function of the material grain size, and a is primarily a function of the grain boundry. Although materials such as silicon carbide also exhibit non-linear resistance, these materials when employed as commercial varistors do not provide exponents greater than 6. The metal oxide varistor employed in the present invention, can provide a in excess of 10, such as 30, within a current density range of to 10 amperes per square centimeter.

The metal oxide material can be more particularly described as a polycrystalline ceramic material of a particular metal oxide, utilized in combination with small quantities of one or more other metal oxides. Some of the metal oxides materials which can be employed in the present invention are shown by Matsuoka US. Pat. No. 3,503,029. In the present invention, the metal oxide found to be most effective is zinc oxide with small quantities of bismuth oxide, while other metal oxides, such as aluminum oxide, iron oxide, magnesium oxide, and calcium oxide can be added in combination with bismuth oxide. The various metal oxides are sintered after being shaped in a particular manner from the metal oxide powder or mixture. The characteristics of the metal oxide as shown by the above equation are defined by the grain crystal size, grain composition, grain boundry composition, the grain boundry thickness, all of which can be controlled in the ceramic fabrication process. The sintered metal oxide varistor can be readily fabricated into a variety of shapes and sizes. As a result it can be precision machined to produce composites by pressure contact between conventional magnetic core material and varistor when properly fashioned to size.

Additional features of the present invention can be further shown by the attached drawings where FIG. 1 shows a composite electromagnetic core of sintered metal oxide and magnetic material. As decore when used as the outer sheath.

FIG. 2 (a) and (b) shows views of the electromagnetic core composite of sintered metal oxide and magnetic material wired to produce inductors in accordance with the invention.

FIG. 3 (a) is a view in section of a self-adjusting autotransformer having a sintered metal oxide layer between the conductor windings and the magnetic core.

FIG. 3 (b) is a sectional view of an autotransformer having sintered metal oxide layers between the conductor windings and exterior to both sides of the magnetic core.

FIG. 4 is a top view of a transformer showing electrodes between conductor coil windings in ohmic contact with a metal oxide varistor as an outer layer of a sintered metal oxide magnetic core composite.

More particularly at FIG. 1, there is shown at 10, a composite core for an electromagnetic device such as an inductor which can have an inner portion at 11 disposed within an outer sheath at 12. The composite core can consist of sintered metal oxide, as defined above, which can constitute either the inner portion or outer sheath, and a magnetic material constituting the remaining mass of the composite core which can be conducting, such as Mo-Permalloy metal, or non-conducting, such as ferrite. In instances where the inductor coils are in contact with metal oxide varistor or conducting magnetic core material, wire insulation can be required depending upon the degree of volts per turn and volts per mil.

An inductor having a composite core of FIG. 1, for example, with an inner mass of metal oxide varistor, which is electroded at both ends to provide ohmic contact between the varistor and soldered terminals, can suppress sparking during sudden voltage surges. Conduction through the varistor can be readily achieved as defined by the above relation where a can be 10 or more. Electroding of the varistor can be effected with a silver-frit which can be silver screened on the surface of the varistor by firing.

In instances where the metal oxide is not electroded it can serve as a thermal conducting means to an external heat sink, as it can be readily fabricated to a variety of shapes.

If employed as the outer sheath on a magnetic core, the metal oxide varistor can again serve as a spark suppressor between coil windings, or if properly electroded, between the terminals. Electromagnetic devices having the composite core of FIG. 1, are more particularly shown in FIGS. 2 (a) and 2 (b).

FIG. 2 (a) shows an inductor 20 having at 21 an inner mass of sintered metal oxide as part of the composite core, a sheath of magnetic material at 22, coil windings at 23, and electrodes 24 and 25 at the ends of the sintered metal oxide. The electrodes provide ohmic contact between the varistor and the conductor and can be applied as a slurry of glass and silver by firing at temperatures of 500 to 850C in an oxidizing atmosphere such as oxygen or air. Contact between the silver electrodes and the conductor can be achieved by conventional soldering techniques. The sintered metal oxide can be initially molded or extruded in the form of a milled metal oxide mixture comprising on the average of from about 96.5 to 97.5 mole percent of zinc oxide and from about 2.5 to about 3.5 mole percent of a mixture of anyone or more of the aforementioned oxides as described above. Sintering of the metal oxide core can be achieved in air at l250 to l450C for l-3 hours. Spark suppression can be achieved between electrodes 24 and 25, when during a sudden voltage surge for example, if the varistor characteristics of the inner sintered metal oxide mass are exceeded conduction will occur in accordance with the previously described relationship where a can exceed 10. Beneficial thermal conduction for achieving more uniform heat dissipation can occur where the sintered metal oxide is not electrically connected to the coil winding. FIG. 2 (b) shows another modification of an inductor, where the sintered metal oxide constitutes the outer sheath and again serves as a spark suppressor. Electrodes are shown at 28 and 29.

FIG. 3 (a) shows more particularly how the sintered metal oxide varistor can be employed as a spark suppressor in a self-adjusting autotransformer, which is shown in toriodal form. At 31, there is shown a sintered metal oxide varistor layer which can be of uniform thickness or tapered in accordance with voltage requirements. A magnetic core is shown at 32 and coil windings at 33. A sweep arm at 34 contacts exposed coil windings insulated on the side on contact with the varistor. Those skilled in the art would know that spark suppression can be achieved by providing a current path across the varistor during voltage surges originating between the sweep arm and an electrode on the varistor not shown.

FIG. 3 (b) shows more particularly an autotransformer at 35, in toroidal form, or which can be rectangular in shape if desired, having an inner magnetic core at 36, which can be conducting or non-conducting, varistor layers at 37 and 38 and coil windings at 39. In contrast to a conventional autotransformer, the electromagnetic device at 35 provides for spark suppression between coil windings in contact with the sintered metal oxide and in instances where the magnetic core is conducting, across both varistor layers.

In FIG. 4, at 40, the top view of a transformer in toroidal form is shown, having a metal oxide varistor layer at 41, six electrodes typified by 42 in ohmic contact with such varistor, conductor coils at 43, and terminal leads typified by 44-47 soldered to such electrodes. If desired, a rectangular shaped composite core connecting path to an external heat sink to urt er dissipate heat generated by such electromagnetic device.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electromagnetic device comprising,

a. a composite core said core comprising a composite of a metal oxide varistor and magnetic material,

b. a conductor wound around said composite core,

c. terminals of said conductor electrically contacting said metal oxide varistor.

2. An electromagnetic device in accordance with claim 1, where said composite core comprises magnetic material in a sheath of metal oxide varistor.

3. An electromagnetic device in accordance with claim 1, where said composite core comprises metal oxide varistor in a sheath of magnetic metal.

4. An electromagnetic device in accordance with claim 1, where the magnetic material is ferrite.

5. An electromagnetic device in accordance with claim 1, where the metal oxide varistor consists essentially of zinc oxide.

6. An electromagnetic device in accordance with claim 1, having electrodes in ohmic contact with metal oxide varistor and terminals soldered to such electrodes.

7. An electromagnetic device in accordance with claim 1, where the metal oxide varistor extends beyond the coil winding and serves as a thermal connecting path to an external heat sink.

8. An inductor in accordance with claim 1, having metal oxide varistor with an a value exceeding 10 as an inner mass within a ferrite sheath, terminal electrodes in ohmic contact with said metal oxide varistor soldered to conductors.

9. A self-adjusting autotransformer in accordance with claim 1, having a metal oxide varistor layer with an a value exceeding 10 between the coil windings and the magnetic core to provide a conducting path between the autotransformer sweep arm and a conductor in ohmic contact to said varistor layer.

10. An autotransformer in accordance with claim 1, having a varistor layer between the coil windings on magnetic core, where said varistor layer provides an a greater than l0 and has electrodes in ohmic contact with conducting leads soldered thereto to provide improved spark suppression between said electrodes during high voltage surges. 

2. An electromagnetic device in accordance with claim 1, where said composite core comprises magnetic material in a sheath of metal oxide varistor.
 3. An electromagnetic device in accordance with claim 1, where said composite core comprises metal oxide varistor in a sheath of magnetic metal.
 4. An electromagnetic device in accordance with claim 1, where the magnetic material is ferrite.
 5. An electromagnetic device in accordance with cLaim 1, where the metal oxide varistor consists essentially of zinc oxide.
 6. An electromagnetic device in accordance with claim 1, having electrodes in ohmic contact with metal oxide varistor and terminals soldered to such electrodes.
 7. An electromagnetic device in accordance with claim 1, where the metal oxide varistor extends beyond the coil winding and serves as a thermal connecting path to an external heat sink.
 8. An inductor in accordance with claim 1, having metal oxide varistor with an Alpha value exceeding 10 as an inner mass within a ferrite sheath, terminal electrodes in ohmic contact with said metal oxide varistor soldered to conductors.
 9. A self-adjusting autotransformer in accordance with claim 1, having a metal oxide varistor layer with an Alpha value exceeding 10 between the coil windings and the magnetic core to provide a conducting path between the autotransformer sweep arm and a conductor in ohmic contact to said varistor layer.
 10. An autotransformer in accordance with claim 1, having a varistor layer between the coil windings on magnetic core, where said varistor layer provides an Alpha greater than 10 and has electrodes in ohmic contact with conducting leads soldered thereto to provide improved spark suppression between said electrodes during high voltage surges. 